Abstract K 2U 2O 7 single crystals have been synthesized from a melt containing uranium, niobium oxides and potassium carbonate. The crystal structure of K 2U 2O 7 has been determined (space group R 3 ̄ m , a = 3.96(2) A ̊ , c = 19.82(4) A ̊ , Z = 3 2 ). This study led to three possible distributions for the oxygen vacancies: on the uranyl bond, on the oxygen atoms of the equatorial plane of the uranyl polyhedron or on both of these two sites. A Mössbauer ( 237Np) study of the isotypic compound K 2Np 2O 7 showed that the vacancies are located on the oxygen positions in the equatorial plane of the neptunyl ion. Mössbauer spectra of CaNpO 4 have been recorded at 4.2 K and 77 K. The isomeric shift value for this compound has been attributed to a neptunium(VI) ion with a coordination number of eight.